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Probe and impedance

Discussion in 'General Electronics Discussion' started by 00135599, Aug 23, 2014.

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  1. 00135599


    Aug 23, 2014
    What's the deal with impedance when connecting measured circuit to oscilloscope? Why do we use pasive voltage probe and how do they work? What else is the problem in measuring signals with oscilloscope? How do we solve it? Are there any helpful videos for this issue? I would use some explanation of impedance when speaking about measurment of signal, sensors, probes, etc.
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
  3. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    The scope probe is made up of usually these components: All of these will vary dependant on the probe. I am talking about the standard scope probe most people use.

    1) Tip capacitance of say 15pf

    2) Capacitor adjustment for waveform purity.

    3) 9 M Ohm resistor near probe tip.

    4) Resistance wire inside coax of a few hundred Ohms

    5) Coax capacitance of 100 pF or so.

    Oscilloscope has 1 M Ohm and 15 pF connected across the input to GND. The scope probe matching is done by the capacitance across the 9 M Ohm resistor which is approx. 15 pF and the internal scope capacitance of the same 15 pF.

    The 9 M Ohm and 1 M Ohm is attenuation for D.C. They call it a X10 probe but it actually attenuates by a factor of 10. If the probe is fitted with a X1 switch this bypasses the 9 M Ohm and parallel 15 pF at the tip and the signal just sees the few hundred Ohm lead resistance and 1 M Ohm scope resistance connected to GND.

    But this gives a lower bandwidth because you have switched out or actually shorted the impedance matching 15 pF on the tip.

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